Receiver and method for decoding of truncated data

ABSTRACT

Coded digital data symbols sent from a transmitter through a transmission channel of a communications network are received in a receiver. An estimate, represented by a first number of bits, of a sent data symbol is calculated, and a second number of bits, lower than the first number, is selected from the estimate to achieve a rounded estimate represented by the second number of bits. The rounded estimate is decoded to achieve a decoded data symbol. A target value for a block error rate of the transmission channel is received from the network; and the second number of bits is selected in dependence on the target block error rate value. Thus an optimal rounded estimate is provided in most situations, and the method can be performed with the limited computational resources of a terminal.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method of receiving coded digital datasymbols sent from a transmitter through a transmission channel of acommunications network, the method comprising the steps of calculatingan estimate of a sent data symbol, said estimate being represented by afirst number of bits; selecting from said calculated estimate a secondnumber of bits, said second number being lower than said first number,to achieve a rounded estimate being represented by said second number ofbits; and decoding the rounded estimate to achieve a decoded datasymbol. The invention further relates to a receiver for receiving codeddigital data symbols sent from a transmitter through a transmissionchannel of a communications network.

DESCRIPTION OF RELATED ART

Terminals for use with such communications networks are normally limitedin size and computational resources. One result of this limitation isthat there is only a limited bit width to represent data values in aterminal. Therefore, it will often be necessary to truncate or rounddata values before they can be handled in the terminal. In order toutilize the available bit width optimally, data values to be processedmay be scaled by multiplying them with a scaling factor before they aretruncated.

One example of data needing to be limited in bit width is found in thereceiver of the terminal. The received signal is here represented as adigital data value having one limited bit width in two-complementnotation in both the real and the imaginary parts. The received signalis then multiplied by the conjugate value of a channel estimate computedin the receiver to obtain an estimate of the sent coded data symbol. Theconjugate value of a channel estimate also has a limited bit width intwo-complement notation in both the real and the imaginary parts, whichmay be equal to or different from that of the received signal. When thetwo values are multiplied the resulting estimate will have a bit widththat is equal to the sum of the two other bit widths. However, the bitwidth available for the processing of this estimate will also be limitedand probably smaller than the sum of the two other bit widths.

Therefore, the bit width has to be reduced before the estimate isfurther processed, and a scaling factor may have to be selected in orderto utilize the available bit width optimally. It is known to find ascaling factor by means of an optimisation algorithm that computes onefixed factor intended to be optimal in the current situation. However,since the signal level in the receiver typically changes very rapidly, afixed factor will not be optimal. Adaptive algorithms that constantlyupdates the scaling factor based on the input signals are also known.Although these adaptive algorithms may be able to provide optimalscaling factors in most situations, they are quite complex and requireconsiderable computational resources, and as mentioned before suchresources are rarely available in the terminals in question here.Although reference is here made to a scaling factor and truncation, thesame problem exists for other rounding methods as well.

Therefore, it is an object of the invention to provide a method of theabovementioned type which can provide an optimal rounded estimate inmost situations, and which can be performed with the limitedcomputational resources of a terminal of the type described above.

SUMMARY

According to the invention the object is achieved in that the methodfurther comprises the steps of receiving from said network a targetvalue for a block error rate of the transmission channel; and selectingsaid second number of bits in dependence on said target block error ratevalue.

Many networks provide information on such a target value, which shouldbe used by the receiver, and by selecting a rounded estimate based onthis target value, a simple method, which requires much less complexitythan the known adaptive algorithms, is achieved. The estimated codeddata symbols, of which the bit width is limited, are used as the inputto a channel decoder, and the performance of this decoder changes withthe selected rounded estimate, e.g. in the form of a scaling factor. Fora given signal-to-interference ratio the obtained block error ratedepends on the scaling factor. When the target value for the block errorrate is known, it is thus simple to select the scaling factor which isknown to provide the best results in the range around this target value.

Although other rounding methods may be used, in one embodiment the stepof selecting said second number of bits comprises the steps ofmultiplying said estimate by a scaling factor; and truncating a numberof bits from said multiplied estimate.

In an expedient embodiment the scaling factor has the form 2^(n), wheren is an integer. The use of this scaling factor corresponds to shiftingthe bit values n bits to the left, and a scaling is achieved whichrequires almost no computational resources.

When the method further comprises the step of selecting said scalingfactor from a stored table comprising corresponding values of saidtarget block error rate and said scaling factor, it is just to selectfrom the table the scaling factor corresponding to the received targetvalue.

When said target block error rate value is the target BLER value definedin the technical specifications of 3GPP (3^(rd) Generation PartnershipProject), a method which is appropriate in terminals according to thesespecifications is achieved.

As mentioned, the invention also relates to a receiver for receivingcoded digital data symbols sent from a transmitter through atransmission channel of a communications network, the receiver beingarranged to calculate an estimate of a sent data symbol, said estimatebeing represented by a first number of bits; select from said calculatedestimate a second number of bits, said second number being lower thansaid first number, to achieve a rounded estimate being represented bysaid second number of bits; and decode the rounded estimate to achieve adecoded data symbol. When the receiver is further arranged to receivefrom said network a target value for a block error rate of thetransmission channel; and select said second number of bits independence on said target block error rate value, a receiver which canprovide an optimal rounded estimate in most situations, and which cancalculate the rounded estimate with the limited computational resourcesof a terminal of the type described above, is achieved.

Although the receiver may use other rounding methods, in one embodimentthe receiver is further arranged to select said second number of bits bymultiplying said estimate by a scaling factor; and truncating a numberof bits from said multiplied estimate.

In an expedient embodiment the scaling factor has the form 2^(n), wheren is an integer. The use of this scaling factor corresponds to shiftingthe bit values n bits to the left, and a scaling is achieved whichrequires almost no computational resources.

When the receiver comprises a stored table comprising correspondingvalues of said target block error rate and said scaling factor, fromwhich table said scaling factor can be selected, it is just to selectfrom the table the scaling factor corresponding to the received targetvalue.

When said target block error rate value is the target BLER value definedin the technical specifications of 3GPP (3^(rd) Generation PartnershipProject), a receiver which is appropriate in terminals according tothese specifications is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described more fully below with reference tothe drawings, in which

FIG. 1 shows a receiver circuit in which the invention may be used,

FIG. 2 shows how an estimate having 16 bits may be truncated so that theeight most significant bits remain,

FIG. 3 shows the situation of FIG. 2, when the estimate has a number ofleading zeros,

FIG. 4 shows a situation in which the leading zeros are not included inthe selected bits,

FIG. 5 shows how the estimate may be multiplied by a scaling factorbefore truncation,

FIG. 6 shows the circuit of FIG. 1 modified with a scaling circuit and atruncation circuit,

FIG. 7 shows the performance of a decoder measured as an achieved blockerror rate (BLER) as a function of the signal-to-interference ratio(SIR) of the received radio signal for two different scaling factors,

FIG. 8 shows a table corresponding to FIG. 7 for selection of thescaling factor,

FIG. 9 shows the performance of a decoder measured as an achieved blockerror rate (BLER) as a function of the signal-to-interference ratio(SIR) of the received radio signal for three different scaling factors,and

FIG. 10 shows a table corresponding to FIG. 9 for selection of thescaling factor.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an example of a receiver circuit 1 in which the inventioncan be used. The receiver circuit 1 is here a part of a terminal, e.g. amobile telephone, for use in a communications system according to the3GPP UMTS standard (3^(rd) Generation Partnership Project—UniversalMobile Telecommunications System). The receiver circuit could also belocated in e.g. a base station corresponding to uplink transmission.

A radio signal representing coded data symbols sent from a correspondingtransmitter is received through an antenna 2 and down-converted to abaseband signal in a front-end receiver block 3. This block furtherincludes a number of other functions, such as amplification, automaticgain control, analog-to-digital conversion, despreading and a RAKEcombiner, but since these functions are not relevant to the presentinvention, they are not described in further detail here.

The output signal from the front-end receiver block 3 is a signal r,which can be described as r=hx+n, where h represents the transmissionchannel, x is the sent coded data symbol, and n represents noise. Allparameters are complex values. The signal r is represented as a digitalvalue in the circuit by a number of bits, e.g. a bits, in two-complimentnotation in the real as well as the imaginary part.

A channel estimator 4 computes a channel estimate ĥ from the receivedsignal r. This estimate is also represented as a digital value in thecircuit by a number of bits, e.g. b bits, in two-compliment notation inthe real as well as the imaginary part. The conjugate function 5calculates the complex conjugate of the channel estimate ĥ, and anestimate y of the sent coded data symbol x is then achieved bymultiplying the received signal r by the conjugate of the channelestimate ĥ, i.e. y=ĥ*r.

The estimate y would now be represented as a digital value by a+b bitsin two-compliment notation in the real as well as the imaginary part,but typically the computational resources are not sufficient to handlevalues with so many bits, and therefore some of the bits have to betruncated so that the estimate y is represented by e.g. c bits, wherec<a+b, before the estimate is further processed in a channel decoder 6.

FIG. 2 illustrates an example, where the received signal r and theconjugate ĥ* of the channel estimate ĥ are both represented by eightbits, i.e. a=b=8. The estimate y will then be represented by 16 bits.If, however, only eight bits are available also for the representationof the estimate, the 16-bit estimate y has to be replaced by an 8-bitestimate y′, and therefore the other eight bits will have to betruncated. Since all 16 bits may be carrying information, it will beobvious to keep the eight most significant bits and truncate the eightleast significant bits, as it is shown in FIG. 2.

However, by low signal levels the most significant bits of the receivedsignal r may have the value “0”, and the same may be the case for theconjugate ĥ*. In FIG. 3 an example is shown, in which the three firstbits of r and the two first bits of ĥ* have the value “0”. As a result,also the five first bits of the estimate y will have the value “0”. Ify′ is still taken as the eight most significant bits of y, as shown inFIG. 3, much information will be lost, since only three informationcarrying bits are left in y′.

Instead, it would be more expedient to leave out the five “0”-bits andselect the following eight bits for y′, as it is illustrated in FIG. 4.The same result is achieved if the value y with the leading zeros isshifted five bits to the left, which corresponds to multiplying y by ascaling factor 2⁵, and y′ is then again taken as the eight mostsignificant bits of the scaled value y_(sc). This is illustrated in FIG.5, and a corresponding circuit is shown in FIG. 6, in which the signal yis scaled in the scaling circuit 8, and a number of bits are truncatedfrom the scaled value y_(sc) in the truncating circuit 7. An overflowcheck and a no-overflow check may be applied to the scaled value y_(sc)to ensure that the value is shifted the optimal number of bits to heleft.

The present invention relates to the selection of an appropriate scalingfactor to be used in the scaling circuit 8. If a fixed factor is used,there will sometimes be overflow, and sometimes there will still beleading zeros. Adaptive algorithms exist which are able to continuouslyupdate the scaling factor based on the input to the circuit, but thesealgorithms typically require more computational power than is availablein portable terminals.

As mentioned, the estimate y′, i.e. the estimated coded data symbol, isthe input to a channel decoder 6, which as examples could be a turbodecoder or a convolutional decoder. The performance of this decoder willdepend on the selected scaling factor in the scaling circuit 8. Theperformance may be measured as an achieved block error rate (BLER) as afunction of the signal-to-interference ratio (SIR) of the received radiosignal. FIG. 7 shows an example, where two different scaling factors,factor I and factor II, are used. The signal-to-interference ratio maybe changed by changing the signal level of the signal transmitted fromthe remote end of the transmission link, and thus FIG. 7 shows as anexample that if scaling factor I is used in combination with thisdecoder, a signal-to-interference ratio of −8.3 dB is needed to providea block error rate of 10⁻². If, however, scaling factor II had beenused, a signal-to-interference ratio of −9 dB would have been sufficientto achieve the same block error rate. On the other hand, if a blockerror rate of 10⁻⁴ is required, scaling factor I is the only usablescaling factor, because with factor II block error rates belowapproximately 10⁻³ cannot be achieved. If, for instance, scaling factorI is 2², corresponding to shifting two bits to the left, and scalingfactor II is 2⁴, corresponding to shifting four bits to the left, factorII will result in overflow when the signal level reaches a certainlevel, and that is the reason that prevents the low block error ratesfrom being achieved with this factor.

In systems according to 3GPP the network actually provides the receiverwith a target value for the block error rate (target BLER), and thusaccording to the invention this target BLER value is used to select thescaling factor of the scaling circuit 8.

In a preferred embodiment a table with corresponding values of targetBLER values and scaling factors is used. An example is shown in FIG. 8.It is seen that scaling factor I (2²) is selected for target BLER values≦10⁻³, while scaling factor II (2⁴) is selected for target BLER values>10⁻³.

Another example is shown in FIGS. 9 and 10, in which three differentscaling factors are used. In this case, scaling factor I (2²) isselected for target BLER values ≦10⁻⁴, while scaling factor II (2³) isselected for target BLER values in the range 10⁻⁴-10⁻³, and scalingfactor III (2⁴) is selected for target BLER values >10⁻.

By using the information about the target BLER value and a look-up tablethe scaling factor can be optimized for several target BLER valuescompared to the situation where only one scaling factor is used. Thebetter performance is achieved with the use of only slightly higheramount of computational resources. Thus the suggested solution requiresmuch less complexity than the use of adaptive algorithms, whichcontinuously estimate e.g. the signal-to-interference ratio or theactual block error rate.

In the above description the information about the target BLER value isused for selecting a scaling factor. Instead of using a scaling factor,the relevant bits may also be selected directly as it was illustrated inFIG. 4, and in that case the information about the target BLER valuedefines the position of the selected bits. Further, truncation is justone of several possible rounding methods, and of course the idea of theinvention can be used with other rounding methods as well.

Although a preferred embodiment of the present invention has beendescribed and shown, the invention is not restricted to it, but may alsobe embodied in other ways within the scope of the subject-matter definedin the following claims.

1. A method of receiving coded digital data symbols sent from atransmitter through a transmission channel of a communications network,the method comprising the steps of: calculating an estimate (y) of asent data symbol, said estimate being represented by a first number(a+b) of bits; selecting from said calculated estimate a second number(c) of bits, said second number (c) being lower than said first number(a+b), to achieve a rounded estimate (y′) being represented by saidsecond number (c) of bits; and decoding the rounded estimate (y′) toachieve a decoded data symbol, characterized in that the method furthercomprises the steps of: receiving from said network a target value for ablock error rate of the transmission channel; and selecting said secondnumber of bits in dependence on said target block error rate value.
 2. Amethod according to claim 1, characterized in that the step of selectingsaid second number of bits comprises the steps of: multiplying saidestimate (y) by a scaling factor; and truncating a number of bits fromsaid multiplied estimate (y_(sc)).
 3. A method according to claim 2,characterized in that said scaling factor has the form 2^(n), where n isan integer.
 4. A method according to claim 2 or 3, characterized in thatthe method further comprises the step of selecting said scaling factorfrom a stored table comprising corresponding values of said target blockerror rate and said scaling factor.
 5. A method according to anyone ofclaims 1 to 4, characterized in that said target block error rate valueis the target BLER value defined in the technical specifications of 3GPP(3^(rd) Generation Partnership Project).
 6. A receiver for receivingcoded digital data symbols sent from a transmitter through atransmission channel of a communications network, the receiver beingarranged to: calculate an estimate (y) of a sent data symbol, saidestimate being represented by a first number (a+b) of bits; select fromsaid calculated estimate a second number (c) of bits, said second number(c) being lower than said first number (a+b), to achieve a roundedestimate (y′) being represented by said second number (c) of bits; anddecode the rounded estimate (y′) to achieve a decoded data symbol,characterized in that the receiver is further arranged to: receive fromsaid network a target value for a block error rate of the transmissionchannel; and select said second number of bits in dependence on saidtarget block error rate value.
 7. A receiver according to claim 6,characterized in that the receiver is further arranged to select saidsecond number of bits by: multiplying said estimate (y) by a scalingfactor; and truncating a number of bits from said multiplied estimate(y_(sc)).
 8. A receiver according to claim 7, characterized in that saidscaling factor has the form 2^(n), where n is an integer.
 9. A receiveraccording to claim 7 or 8, characterized in that the receiver comprisesa stored table comprising corresponding values of said target blockerror rate and said scaling factor, from which table said scaling factorcan be selected.
 10. A receiver according to any one of claims 6 to 9,characterized in that said target block error rate value is the targetBLER value defined in the technical specifications of 3GPP (3^(rd)Generation Partnership Project).